The invention pertains to a method for ultrasonic imaging, particularly of moving bodies, such as tissues, flows, or the like, which includes the following steps:
Periodically emitting ultrasonic pulses along a predetermined view line and with a predetermined repetition rate through one or more transducers;
Receiving the echoes produced by the body and/or the tissues or flows and transforming them into echo signals;
Processing the echo signals for extracting information therefrom and for generating an image based on such information.
Particulalrly the invention relates to an apparatus for implementing and using the method and to an apparatus for detecting intraoperative surgical instruments and calcifications or similar biological structures
The ultrasonic imaging technique consists in generating an ultrasonic pulse beam, having frequencies in the RF range, from a set of aligned emitting transducers to illuminate or soundproof a definite section of a body or of a body part to be examined. As the individual pulses propagate in the body according to a predetermined penetration depth, as determined by their nature and by the nature of the body being examined, they are reflected by the structures forming the body and generate reflected echoes, which are detected by detectors and transformed into echo signals. The different structures or types of tissues encountered by the illuminating pulses while the latter propagate in the body along a propagation axis or view line produce changes in the emitted carrier, in the form of amplitude modulations or phase variations. These changes are the information to be extracted by the echo signals to obtain the ultrasonic image.
In the simplest form of ultrasonic image, the so-called B-mode image, a black and white image is generated, in which the different tones of gray, black and white are related to the intensity or the amplitude of the received echo signal. Processing requires synchronization, on a predetermined time base, of the emitted pulses and of the received echo signals to accurately reconstruct the zones wherefrom the echoes arrive along the propagation line, in accordance with the propagation of the illuminating pulse and further to allow correlation of the echo signals to their respective illuminating pulse-emitting transducers.
Therefore, the information contained in the echo signals may be interpreted either with respect to amplitude modulations of the illuminating pulse, as in the so-called B-Mode, or with respect to phase variations, such as in the so-called Doppler, Power Doppler, Color Doppler, or the like.
From surgical point of view there is a need to control position and displacement of intraoperative utensils in the human or animal body by means of a non invasive technique which is capable of giving an intelligible image of the region where the intraoperative utensil is placed.
From the diagnostic point of view, there is the need of noninvasively imaging particular tissue types, such as for example calcifications or similar biological structures in the human and animal body. This kind of tissues are not simple to be scanned with ultrasound techniques.
From the diagnostic point of views there is also a very high interest in the ultrasonic imaging of flows in the human bodies, particularly of blood flows. The problem consists in that most of the physiological liquids, such as blood, urine, bile, cyst contents, etc. are anechogenic, i.e. transparent, or only partially reflect ultrasounds.
Thus, ultrasonic imaging with the conventional B-Mode method is not feasible and produces poor, unusable results. Therefore, several techniques have been implemented to detect flows, particularly blood flows, with ultrasound apparati, the so-called Doppler, Power Doppler, Color Doppler techniques, or the so-called Harmonic Imaging technique, in which the echo signal is examined with respect to its harmonics. These prior art techniques require both processing in the frequency or phase domain, which add a considerable processing burden, and the use of the so-called contrast agents, consisting of microbubbles, which, when injected in spontaneous flows, have a hyperechogenic behavior.
Actual techniques for imaging of physiological flows are not optimized for imaging of surgical apparatus or for biological structures such as calcifications. Indeed imaging apparatus and techniques normally are arranged and chosen in such a way as to avoid the contributes of the scattered ultrasonic beams due to the presence in the region being scanned of surgical intraoperative utensils or biological structures such as calcifications.
The present invention has the object to provide a method for ultrasonic imaging, particularly of moving bodies, such as intraoperative utensils, tissues, flows or the like, which allows fast real time and simple imaging of the said utensils and or tissues and also real time imaging of body flows even without using contrast agents, without excluding the combined use thereof, and at the same time is highly sensitive to movement, thereby allowing to considerably simplify processing of echo signals for extracting and displaying information.
The invention achieves the above purposes by providing a method for ultrasonic imaging, particularly of moving bodies, such as surgical inytraoperative utensils, tissues, flows, or the like, which includes the following steps:
determining a real time vector difference between the echo signals of two pulses successively emitted at predetermined time intervals;
using said difference signal as an information signal for ultrasonic imaging.
Therefore, a direct and vector difference between the received RF echo signals is determined, which involves suppression of the contributions or portions of the echo signals produced by the stationary parts of the body, tissues or the like, whereas the portions of the echo signals produced by moving bodies, such as intraoperative surgical utensils, calcifications or the like and/or tissues or elements, such as red blood cells, etc. give non-zero contributions due to displacement of said moving bodies or parts.
Processing of the difference signal for imaging is performed as in conventional ultrasonic imaging techniques.
The method of the invention allows ultrasonic imaging of moving bodies, particularly of spontaneous body flows, i.e. of blood flows or the like, without using radiopaque agents.
The method of the invention is highly sensitive to movement, therefore it can generate images of very slow or low flow rate flows or of parts which perform micromovements.
The resulting images have a high definition and a considerable level of contrast and, in combination with high sensitivity to movement and micromovement of bodies in the operating range of the probe, they allow to display and monitor movements of invasive surgical instruments, such as microinstruments, intraoperative probes, needles, etc. better than the other prior art ultrasonic methods.
Thanks to the high sensitivity of the method, ultrasonic images may be also obtained from biological structures which are highly echogenic, but perform little or very slow movements, i.e. perform even slow micromovements, such as calcifications or breast cancer formations.
In order to adapt the imaging technique according to the invention to the different kind of analysis needed, the difference RF signal is further treated by non linear filtration of the signal components which are not of interest for the specific analysis and may hinder correct evaluation of the interesting part of the echo signals.
Indeed high echogenic tissues or materials may contribute very significatively to the echo signals and with such an high response that the contribution to the signal due to the tissue of interest is reduced to a very insignificant part of the signal.
Due to the said sensitivity of the method according to the invention to micromovements, high ecogenic materials, such as in the case of surgical utensils or calcifications may generate some sort of intensity flashes that let the contribution of less ecogenic tissues practically disappear This may be a problem when the flow to be detected, e.g. blood flow, is particularly anechogenic, and is surrounded by hyperechogenic, i.e. hyperreflecting or hyperdense tissues, with respect to ultrasonic pulses. In this case, the micromovements between the probe and said hyperechogenic tissues (e.g. walls of blood vessels or tissues of organs), give a non-zero difference signal, whose intensity is much higher than the contribution, for instance, of the blood flow or other flows or moving bodies, which are less echogenic or even classifiable as anechogenic, whereby the signal generated by the flow has a limited level with respect to the signal produced by echogenic or hyperechogenic tissues. In this case, the method may fail to image said flows.
From one point of view the method according to the invention allows to rapidly scan and image intraoperative surgical utensils and biological structures such as calcifications or breast cancer by executing a non linear filtering of the received difference echoes signals which filtering cut away the signal below a certain intensity level, thus using for reconstructing the image merely the high intensity difference signals. This is exactly the contrary which is done in the actual known techniques, since in these actual techniques this high signal components are filtered away or reduced in intensity.
On the other hand in order to image physiological flows the same non linear filtering process is used to neutralize signal flashes generated by relative micromovements between the prove and the echogenic or hyperechogenic tissues which surround the flows whereof ultrasonic imaging is desired and/or by intraoperative utensils or by the said biological structures as calcifications. In this case the part of the difference signal used for generating the image is the part below the above mentioned intensity threshold. In this case there are cut away the difference signal parts above the said intensity threshold.
Particularly for both cases, the above non linear filtering is based on the fact that, in the so-called B-Mode echo signal processing (generally known to the skilled in the art and used substantially in all ultrasound machines) the flows generally composed of anechogenic components give much lower signal powers or intensities than echogenic and hyperechogenic tissues.
Therefore, a maximum intensity threshold may be defined, above which the components of the difference signal between two successive echoes are drastically attenuated or set to zero, while the components of the difference signal between successive echoes whose intensity or power is below said threshold are retained, or vice versa depending on the kind of analysis is being executed.
With reference to a further characteristic of the invention, it is possible to define more than one unique intensity threshold of the echo difference signals. In this case, in addition to a maximum threshold, a minimum threshold may be defined, i.e. a window, whereby the signals or signal components included in said intensity window may be alternatively retained, or drastically reduced or set to zero, and the components of the difference signal whose intensity is outside said window or range are strongly attenuated or set to zero or alternatively retained.
According to the above feature of setting more than one threshold different combination of intensity components of the difference echoes signals may be used for reconstructing an image, being thus able to add information to the image reconstructed relating to different tissues which may surround the object scanned and whose image is required.
By further applying weights to the signal components which are due to tissues or objects which are not o principal interest of imaging it is possible to scale the contribution of this tissues or bodies in order to enhance the image of the relevant object and give at the same time a view of its position in the region scanned relatively to other tissues or objects, which will appear in the image with a level of intensity adaptable to the desires of the user.
Besides determining the maximum signal intensity threshold above which echo signals hate to be set to zero or retained for constructing the image, a limited-intensity imaging of the background of the whole section may be allowed, i.e. of the zone in the proximity or surroundings of the spontaneous flow, particularly the blood flow.
In this case, from a first echo signal, only a portion of the subsequent echo signal is subtracted. By this arrangement, contribution by stationary tissues or parts of the body structure is not completely suppressed but generates a signal greater than zero related to said stationary structures or tissues, hence a limited-intensity image of the zones surrounding the intraoperative utensil, the biological structure or the flow under examination.
Thanks to this arrangement, it is not only possible to obtain real time images of intraoperative utensils, radiopaque biological structures and/or anechogenic flow, but also of the tissues or regions surrounding such flow, concurrently illuminated by the pulses emitted by the transducer.
It is important to understand that the present invention uses a signal components of the echo signals which normally are cut or filtered away or strongly attenuated as being considered of damage to the reconstruction of an image. It is also important to consider the fact that the particular non linear filtering allows parallel evaluation of the signal components above and below the intensity threshold thus allowing a parallel real time imaging of ecogenic bodies or tissues executing micromovements relating to the ultrasound probe and of not echogenic bodies or tissues executing spontaneous displacements or flows in the region scanned. From the above description it appears clearly that the method according to the invention allows imaging of a intraoperative utensil and at the same time of blood flow in the vicinity of the utensil, allowing to combine these two images and further to combine these images alone or together with an image of the surrounding tissues.
Defining more than one threshold of intensity it is further possible to set more levels of combination and of scaling of the combined images which may be obtained, controlling their contribution to the final image in a simple way.
Furthermore it is important to note that according to the filtering method suggested by the present invention, i.e. a non linear filtering due to a signal cut-off by means of a discrimination threshold, it is possible to implement the method comprising the difference and anti-clutter filtering of the signals using only two shots or bursts for each vector. On the contrary the actual techniques using digital filters of the type denominated FIR, need in their most simple embodiments at least three shots or bursts for each vector. This feature allow to operate with the method according to the invention at higher frame rates than according to the actual methods.
The invention also pertains to an apparatus for implementing said method, which includes
a probe for emitting RF ultrasonic pulses, generally consisting of a set of transducers aligned on at least one line;
transducers for receiving the echo signals reflected by the body under examination while the illuminating ultrasonic pulses propagate therein;
processing means, including means for generating an image from the received signals;
means for determining a difference signal between two received echo signals.
Moreover, the invention provides means for generating a threshold for filtering the signal useful for imaging, which is related to signal intensity.
Weighting means may be provided for the second signal which is subtracted from the first echo signal, to generate a partial subtraction of said second signal from the first echo signal.
The characteristics of the invention and the advantages derived therefrom will appear more clearly from the following description of a non limiting embodiment, illustrated in the annexed drawings.